Allow useage of server IDs as well as names.
Allow useage of server IDs as well as names.

RAX have some hidden images useful for build a xenserver host.
Ensure nodepool can use these by referring to them by image UUID.

Change-Id: Idfefc60e762740f4cffa437933007942a0920970

Tesora/tesora-nodepool,Tesora/tesora-nodepool,citrix-openstack/nodepool,seandst/nodepool,seandst/nodepool,citrix-openstack/nodepool